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What is the difference between PWM and PFM ?

November 22, 2023

What is the difference between PWM and PFM?

PWM and PFM

①Pulse width modulation (PWM)

The pulse width modulation (PWM) switching voltage stabilizing circuit adjusts its duty through voltage feedback while the output frequency of the control circuit remains unchanged. ratio, thereby achieving the purpose of stabilizing the output voltage.

②Pulse frequency modulation (PFM)

Pulse frequency modulation (PFM) modulates the signal’s frequency with the input signal amplitude, while its duty cycle remains unchanged. Since the modulation signal is usually a square wave signal with changing frequency, PFM is also called square wave FM.

③Pulse width frequency modulation (PWM-PFM)

 

The performance characteristics of each type are different:

1. Efficiency under heavy load and light load

2. Load regulation

3. Design complexity

4. EMI/noise considerations

Integratedconvertersolutions combine these two modes of operation to take advantage of their respective advantages:

①PWM is the change of wide and narrow frequency, and PFM is the change of presence and absence of frequency.

②PWM uses the pulse width to control the output, and PFM uses the presence or absence of the pulse to control the output.

③PWM-PFM has the advantages of both PWM and PFM.

 

Characteristics and comparison of switching power supply control technology:

The DC/DC converter boosts or steps down the voltage by switching synchronously with the internal frequency, and controls it by changing the number of switches to obtain an output voltage that is the same as the set voltage.

During PFM control, when the output voltage reaches above the set voltage, the switching will stop. The DC/DC converter will not perform any operation until it drops to the set voltage. But when the output voltage drops below the set voltage, the DC/DC converter will start switching again to bring the output voltage to the set voltage.

PWM control also switches synchronously with the frequency, but when it reaches the boost set value, it will try to reduce the current flowing into the coil and adjust the boost to be consistent with the set voltage.

PWM/PFM switching control The DC/DC converter is controlled by PWM when the load is heavy, and automatically switches to PFM control when the load is low, that is, it has the advantages of PWM and PFM in one product. In systems with standby mode, products using PFM/PWM switching control can achieve higher efficiency.

Compared with PWM, the output current of PFM is small, because the DC/DC converter controlled by PFM will stop operating when it reaches the set voltage or above, so the current consumed will become very small. Therefore, reduction in current consumption improves efficiency at low loads.

Although PWM is less efficient at low load, its ripple voltage is small and the switching frequency is fixed, so it is easier to design a noise filter and eliminate it. Noise is also simpler.

PFM:

CONSTANT FREQUENCY RAMP

At light loads, the PWM converter automatically switches to a “low power” mode to minimize battery current consumption

This mode is sometimes referred to as a “PFM” – but is actually a fixed frequency (PWM) converter that intermittently switches on and off.

PWM:

PWM

Power supply ripple PWM VS PFM

PFM VS PWM

Advantages and Disadvantages of Switching Power Supply Control Technology

The main advantage of PFM compared to PWM is efficiency:

For PFM and PWM with the same peripheral circuits, the peak efficiency of PFM is equivalent to PWM. But before the peak efficiency, the efficiency of PFM is much higher than that of PWM, which is the main advantage of PFM.

Due to the influence of the error amplifier of PWM, the loop gain and response speed are limited, while PFM has a faster response speed.

Compared with PWM, the main disadvantage of PFM is the difficulty of filtering:

Difficulty filtering (harmonic spectrum is too broad).

Before the peak efficiency, the frequency of PFM is lower than the frequency of PWM, which will cause the output ripple to be larger than that of PWM.

PFM control is more expensive than PWM control IC.

The PWM control method is easy to implement, but the PFM control method is not easy to implement.

Charge pump

DCA

A charge pump, also known as a switch capacitor voltage converter, is a type of voltage converter that utilizes so-called “flying” or “pumping” capacitors ( Rather than inductors or transformers) to store energy DC-DC (converter).

Key applications include driving white light LED and milliwatts for backlighting in mobile phones Number of range processors.

Charge pump (switched capacitor) ICs perform DC/DC voltage conversion by utilizing a switching network to power or de-energize two or more capacitors. A basic charge pump switching network constantly switches between powering and powering off the capacitor. C1 (charging capacitor) transfers charge, while C2 (charging capacitor) stores charge and filters the output voltage.

Additional “flying capacitors” and switch arrays provide multiple benefits.

Charge pump ICs can be used as inverters, splitters or boosters. The inverter converts the input voltage into a negative output. When used as a splitter, the output voltage is a fraction of the output voltage, such as 1/2 or 2/3. When used as a booster, it can bring a 1.5X or 2X gain to the I/O. Many portable systems use a single lithium-ion battery or two metal hydride nickel batteries. So when operating in 2X mode, the charge pump can supply the appropriate forward voltage to white LEDs that typically operate in the 3.3V to 4.0V range.

The basic charge pump lacks regulation circuitry, so virtually all charge pump ICs in use today add linear regulation or charge pump modulation. Linear trim has the lowest output noise and provides better performance at lower efficiency. Since the adjustment IC does not transmit the transistor in series, the charge of the control switch resistance Pump modulation can provide higher efficiency and provide more output current for a given chip area (or consumption).

The charge pump eliminates the magnetic fields and electromagnetic interference carried by inductors and transformers. However, there is still a possible source of minor noise, which is the high charging current flowing into the flying capacitor when it is connected to an input source or to another capacitor with a different voltage. Likewise, a “splitter” charge pump can improve efficiency on an LDO without the complexity of an inductive buck regulator.

Forward and flyback

Forward and flyback

Flyback type: Flyback switching power supply refers to a switching power supply that uses a flyback high-frequency transformer to isolate the input and output circuits. “Flyback” means that when the switch is turned on, when the input is high level, the series inductor in the output line is in a discharge state; conversely, when the switch is turned off, when the input is high level, the output The series inductance in the line is in a charged state.

Working principle:

The primary and secondary windings of the transformer have opposite polarities, which is probably where the name Flyback comes from:

When the switch tube is turned on, the primary side inductor current of the transformer begins to rise. At this time, due to the relationship between the secondary terminal and the same terminal, the output diode is cut off, the transformer stores energy, and the load is provided with energy by the output capacitor.

When the switch tube is turned off, the induced voltage of the primary inductor of the transformer is reversed. At this time, the output diode is turned on, and the energy in the transformer supplies power to the load through the output diode, while charging the capacitor to replenish the energy just lost.

The evolution of the flyback circuit:

Can be viewed as an isolated Buck/Boost circuit:

CIRCUIT

In the flyback circuit, in addition to realizing electrical isolation and voltage matching, the output transformer T also has the function of storing energy. The former is an attribute of the transformer, and the latter is an attribute of the inductor, so some people call it an inductive transformer. Sometimes I also Call him an asynchronous inductor.

 

Forward power supply

The transient control characteristics and output voltage load characteristics of the output voltage of the forward transformer switching power supply are relatively good. Therefore, the operation is relatively stable and the output voltage is not prone to jitter. In some situations where the output voltage parameters are relatively high, frequently used.

 

The so-called forward transformer switching power supply means that when the primary coil of the transformer is being excited by the DC voltage, the secondary coil of the transformer has power output.

power output1

Single-ended forward:

power output2

Double tube forward type:

power output3

As can be seen from the above three pictures, the flyback transformer can be regarded as an inductor with a voltage transformation function and is a buck-boost circuit. The forward transformer only has the function of transforming voltage, and the whole can be regarded as a buck circuit with a transformer. The secondary side is connected to the negative end of the first rectifier diode and is connected to the electrolytic capacitor, which is counterattack, and the one connected to the inductor is forward.

 

Generally speaking, the working principle of forward and flyback is different. In forward, the primary works and the secondary also works. When the secondary does not work, the freewheeling inductor freewheels, usually in CCM mode. The power factor is generally not high, and the input and output are proportional to the ratio duty cycle. Flyback is the primary operation, the secondary is not working, and both sides are independent. Generally, in DCM mode, the theoretical power factor is unit power factor, but the inductance of the transformer will be relatively small, and an air gap needs to be added, so it is generally suitable for small and medium power situations. Generally All power supply books will have specific introductions and design formulas.

 

The forward transformer is ideal and does not store energy. However, since the excitation inductance (Lp) is limited, the excitation current makes the core B larger. In order to avoid flux saturation, the voltage transformation requires an auxiliary winding for flux reset; flyback transformer The working form can be regarded as a coupled inductor; the inductor first stores energy and then releases energy. Since the input and output voltages of the flyback transformer have opposite polarities, when the switch tube is turned off, the secondary can provide a reset voltage to the magnetic core, so the flyback transformer does not need to add an additional flux reset winding.

 

main difference

The main difference between forward and flyback is that the high-frequency transformer works differently but they are in the same quadrant. Forward excitation means that when the primary switch tube of the transformer is turned on, the energy is transferred to the load at the same time. When the switch tube is turned off, the energy of the transformer is demagnetized through the magnetic reset circuit. Flyback is the opposite of forward. When the primary switch is turned on, it stores energy in the transformer. But the energy will not be added to the load. When the switch tube is turned off, the energy of the transformer is released to the load side. In a forward switching power supply, the diode at the back is a freewheeling diode. Generally, an additional energy storage inductor is added to the output part. The most important difference between forward and flyback is that the primary and secondary phases of the transformer are inverted.

 

The biggest difference

The biggest difference between forward and flyback operation is that when the switch tube is turned off, the forward output mainly relies on the energy storage inductor and freewheeling diode to maintain the output, while the flyback output mainly relies on the energy released by the secondary of the transformer. Maintain output. The forward circuit is not suitable for multiple outputs. If the forward circuit uses pulse width adjustment for voltage stabilization, the inductor must be connected in series after the secondary rectification. Otherwise, the output voltage is mainly determined by the input and has little effect on the pulse width. The pulse width only affects the output pattern. Wave. There are problems in principle when using a forward circuit for multiple outputs: If no inductor is used for each output, there will be no voltage stabilization effect on input changes, and there will be no safety that a switching power supply should have. If an inductor is added to each circuit: then the output voltage is theoretically related to the load size, and a circuit that does not participate in feedback is not correct.

 

In principle, the flyback circuit is suitable for multi-channel output voltage stabilization. The flyback circuit first stores energy, and then supplies energy to each channel according to the voltage ratio of each channel. First, it can be considered that the output ratio of each channel is unchanged (see below for actual errors). According to the current, who needs more is given more. Principle distribution.

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